The present invention relates in general to power converters including inverters for an electric drive system of an electrified vehicle, and, more specifically, to variation sequences of a pulse-width modulation signal which reduce noise and vibrations associated with the electric drive system.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC linking capacitor. An inverter is connected between the main buses for the DC link and a traction motor in order to convert the DC power to an AC signal that is coupled to the windings of the motor to propel the vehicle.
The inverters include transistor switching devices such as insulated gate bipolar transistors (IGBTs) connected in a bridge configuration having a plurality of phase legs. A typical configuration includes a three-phase motor driven by an inverter with three phase legs. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor. The inverters is controlled in response to various sensed conditions including the rotational position of the electric machine (for determining speed) and the current flow in each of the phases (for determining torque production).
The inverter for the motor may preferably pulse-width modulate the DC link voltage in order to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. Pulse Width Modulation (PWM) control signals applied to the gates of the IGBTs turn them on and off as necessary so that the resulting current matches a desired current. To achieve the desired sinusoidal output voltages, the gate control signals are modulated by a carrier signal to produce a series of pulses that turn inverter switches on and off subject to a duty cycle that is derived by comparing an output of the motor to a desired output. The carrier signal is at a frequency that is higher than the frequency of the sinusoidal signal delivered to the motor so that the inverter output can be accurately controlled. Within the range of acceptable carrier frequencies, the use of a lower frequency is associated with reduced switching losses in the inverter. However, switching at frequencies within the audible spectrum can produce unpleasant high pitched whining and whistling noises that irritate automobile operators and passengers. Thus, some attempts to reduce audible noise have relied on carrier frequencies above the audible spectrum, but these have resulted in higher switching losses.
U.S. Pat. No. 8,907,604, Miller et al, entitled “PWM Frequency Pattern Optimization for NVH” discloses a pseudorandom variation of the PWM carrier frequency to optimally spread PWM energy and reduce audible noise. Spreading the noise energy between multiple carrier frequencies achieves sufficient noise reduction to allow the carrier frequencies to lie within the range of audible frequencies, whereby the increased switching losses associated with higher frequencies can be avoided. Nevertheless, it would be desirable to provide further optimization of noise and switching losses if possible.